FR2571150A1 - CIRCUIT FOR REMOVING SLOW MOBILE ECHOES FOR RADAR DOPPLER - Google Patents

Abstract

IN AMBIGUS FREQUENCY RADARS USING THE DOPPLER EFFECT TO ELIMINATE ECHOES FROM THE GROUND, ECHOES FROM LAND VEHICLES MAY MIX, BY FREQUENCY REFOLDING, WITH USEFUL ECHOES FROM AIRCRAFT. THE OBJECT OF THE CIRCUIT OF THE INVENTION IS TO ELIMINATE THESE ECHOES FROM LAND VEHICLES. THIS CIRCUIT INCLUDES A C1 SWITCH ALLOWING THE CONNECTION FOR EACH RECURRENCE FREQUENCY, ANTENNA RECEPTION CIRCUITS E1 TO A SUMMATION REGISTER F1 TO F4. A CS SUMMATION CIRCUIT CONNECTED TO THE OUTPUTS OF THE SUMMATION REGISTERS F1 TO F4, PERFORMS A POST-INTEGRATION OF THE SIGNALS DETECTED AND PROVIDES AN INTEGRATION SIGNAL. A COINCIDENCE CIRCUIT AND ALSO CONNECTED BY THRESHOLD CIRCUITS A11 TO A14, TO THE OUTPUTS OF THE SUMMATION REGISTERS F1 TO F4, PROVIDES AN INHIBITION SIGNAL ALLOWING TO MASK THE INTEGRATION SIGNAL WHEN EACH SUMMATION REGISTER WITH A LEVEL DELIVERS A SIGNAL GREATER THAN A DETERMINED S2 THRESHOLD.

Description

257 1150

  SLOW MOBILE ECHO REMOVAL CIRCUIT

FOR RADAR DOPPLER

  The invention relates to a circuit for the elimination of echoes of slow mobiles

  for DOPPLER radar applicable in particular to airborne radars.

  In ambiguous frequency radars using the Doppler effect to eliminate ground echoes, terrestrial vehicle echoes can become confused with useful aircraft echoes. The object of the circuit of the invention is to eliminate these echoes of land vehicles. - In unambiguous frequency radars, the elimination of terrestrial vehicle echoes takes place directly by eliminating a noppler zone covering the spectrum of ground echoes and terrestrial vehicle echoes. This zone, already important for non-ambivalent frequency radars,

  becomes prohibitive for frequency-related radars.

  A conventional solution is to track these echoes, that is to say to carry out these echoes antenna tower tower antenna, which requires very important calculation means, given the very high number echoes of land vehicles received by a radar in standby,

  particularly for an airborne radar seeking to detect airborne

  flying at low altitude. On the other hand, this tracking is made difficult by the proximity of these echoes between them (roads) and by the many masks

  encountered at ground level.

  Therefore, the invention relates to a circuit for eliminating the echoes of land vehicles during a single scan (without correlation of sweeping balayaoe) while preserving echoes ambiguous aircraft speed. Incidentally, this device eliminates echoes of second

(or nth) scan.

  The invention therefore relates to a circuit for the elimination of slow mobile echoes for Doppler radar comprising antenna reception circuits providing on a link the digital value of the detected signal, characterized in that it comprises

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  a first switch having an input connected to the output link of the reception circuits and a number of outputs equal to the number

  frequencies of recurrence different from the radar the oppler, said commutation

  to connect its input to the output corresponding to the current recurrence frequency; a summation register connected to each output and providing on an output a detected signal value; a summation circuit connected to the various outputs of the summing registers performing a post-integration, on the time of passage of the antenna beam on the target of the detected signals to provide on an output an integration signal - a cotncidence circuit having as many inputs as there are values of the recurrence frequency each of these inputs being connected to an output of a summation register so that

  coincidence circuit provides an inhibition signal on an output of inhi-

  bition when each summing report delivers a higher level sicnal to a determined threshold - an output circuit having a first input connected to the output of the summing circuit, a second input connected to the muting output of the coincidence circuit, and an output circuit enabling, in the absence of an inhibit signal on the second input, to retransmit any integration signal received on the first input, on an output to

  display circuits, this circuit also makes it possible to prohibit the retransmission

  mission of the integration signal in the event of presence of a muting signal on the second input; and a control circuit providing control signals to the

  switch to summation registers and summation circuit.

  The different objects and features of the invention will appear

  more clearly in the following description made with reference to the

  attached figures which represent: - Figure 1, an embodiment of the circuit of the invention for discriminating amplitude variations: - Figure 2, an embodiment of the circuit of the invention to make a Phase variation discrimination and amplitude variation discrimination:

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  FIG. 3, an exemplary embodiment of a variant of the circuit of the

figure 1.

  - Figure 4, an example of a Doppler filter template.

  One of the fundamental problems of airborne V / CM (Mobile Target Visualization) or MTT (Movinq Tarqet Indicator) radars is the elimination of slow land vehicles whose Doppler frequency is superimposed on the ambient Doppler frequency of air targets. which can not be eliminated by Doppler filtering. The solutions of raising the speed ambience by a "bipulse" type process lead to a considerable increase in the complexity of the hardware in terms of transmission and signal processing. A simpler method is to perform a scanning sweep correlation of the radar map: the difference in positions between two successive sweeps is more important for fast vehicles than for slow vehicles where possible discrimination. However, this assumes that these echoes appear with each scan, which is rarely the case for echoes at ground level (problem of masking by trees, buildings, relief, etc ...). This solution is therefore insufficient and it is necessary

  to eliminate slow echoes during a single antenna passaae.

  With a PFR (Frequency Repetition Rate) radar, the relatively low recurrence frequency (for example 3000 HIz) implies having a noppler filter mask, as shown in FIG. 4, whose ratio between the rejection zone and the passing area is high (40%). To prevent a large number (40%) of the targets from having its ambiguous frequency fall into the blind zone and to be re-fired, the recurrence frequency FR is varied during the illumination time of the target, which displaces the ambient frequencies. and allows the detection of each air target for

at least one value of FR.

  Since the non-ambulatory Eoppler frequencies are brought around the Doppler frequency of the ground echoes, it will be possible to distinguish two groups of targets: - the targets whose unambiguous Doppler frequency is lower than FR which will not be disturbed by the variations of FR : these slow 257f150 targets (velocities <40 m / s above ground echoes) will be assumed to be land vehicles - fast targets with non-ambulatory Doppler higher than FR and

  whose frequency will depend on FR.

  This discrimination will only be possible if there is at most one target in

  each cell of resolution, which is practically the case after eliminating

  fixed echoes by the Doppler filter.

  This discrimination is made, during a passage on the target: - by measurements of amplitude variations at the output of the Doppler filter,

  - and by measurement of phase variations for the different frequencies

quences of recurrence.

  Measurement discrimination of amplitude variations allows about 90% of slow vehicles to be removed (if their power is at least dB above the detection threshold) with a risk of less than a few

  for hundreds to eliminate a useful target.

  Discrimination by phase-shift measurements eliminates the risk of an unnecessary elimination of useful echo (for echoes whose power is at least 3 dB above the detection threshold),

  while improving the elimination of slow vehicles.

  Doppler radars ambiguous in frequency (in speed) use the frequency of recurrence frequency during the time of passage of the antenna on the target to eliminate the holes of detection due to the speeds "blind" (zones of frequencies corresponding, by folding at the repair area

Doppler filter).

  It follows that an ambiguous echo in frequency has an ambient frequency which can be in the rejection zone of the Doppler filter or on the contrary in the pass area of the filter, according to the value of the frequency

of recurrence (FR).

  The N different recurrence frequencies can be suitably chosen so that an ambide echo is in the Doppler filter bandwidth for at least two and at most N-1 values of the recurrence frequency. An unambiguous echo is either in the rejection zone or in the pass zone regardless of the frequency of recurrence (so for N

frequency values).

  An echo of second chanaya balances of distance cells at each FR switching, so it is present, for a cell distance

  given, for up to a value of FR.

  By eliminating the echoes present for the value of FR or N values of FR, the unambiguous echoes which correspond to the range of

  frequency of land vehicles and echoes of nth sweep.

  This device is completed by a measurement of the phase rotation of

  the recurrence recurrence echo.

  When the echo is unambiguous in frequency, this rotation is

almost independent of FR.

  On the other hand, for an ambiguous echo, this rotation depends a lot on FR. Comparing this rotation for the different FR values, it is

  possible to discriminate ambicus echoes and unambiguous echoes.

  Practically, to fill the holes of speed, we have been led to choose for example the following frequencies of recurrence: F1 = 3000 Hz F2 = 3200 Hz F3 = 3400 Hz

F4 = 3600 Hz.

  But it is obvious that these values are only given as an example just like the other numerical values we provide.

in the following description.

  In a foppler radar, the reduced frequency fi at the frequency FR = Fi is obtained from the Doppler frequency fd (measured with respect to the ground echoes) and by the formula: fd fd modulo F. fi = decimal part (- ) oFo! F. The ambient frequency is then: fdi = fiXFi An echo at the frequency fd will be seen for the recurrence frequencies such that fi is in the pass area of the filter, we will then say that the

  number a. is 1 (and zero otherwise).

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  For each frequency, we can calculate the numbers al, a2, a3, a4 which

  indicate the frequencies of recurrence where there will be possible detection.

  It is known to establish a relationship between fd and ai for useful targets, that is to say practically up to 750 m / s or Mach 2.5 at low altitude: (O fd <47.5 KHz). This speed is the speed of the target relative to

  ground, the speed of the carrier is eliminated by a ground speed loop.

  It can be seen that: - all velocity holes, greater than FR, are filled (at least 1 ai non-zero) except the following zones: 2880 to 3600 Hz, ie 720 Hz 6480 to 6600 Hz, or 20 Hz 47400 to 47520 Hz, or ] 20 Hz However, the flanks of the Doppler filter are not stiff, the holes that are observed are partially filled: it was not jusqé useful to

  add a 5th frequency to count residual blind zones.

  Areas for which all have. are at 1 are found mainly

  lently concentrated at low frequencies. In particular, terrestrial vehicles such as fd <2400 Hz, correspond to: -all a. = 0 from 0 to 600Hz i

  - all the a. = 1 from 720 to 2400 Hz.

  FIG. 1 represents a discrimination circuit using measurements of

amplitude variations.

  In this figure, there are antenna reception circuits E] comprising in particular a FD Doppler filter and an envelope detection and normalization circuit intended to solve the false alarm. This detector provides on an output link] 0 values of levels of sional as digital signals. A switch whose input is connected to the link 10 makes it possible to connect it to an output. 1 to 4 each corresponding to a recurrence frequency. Each of these outputs is connected to a summing register whose lonquer is that of the number of quanta distance operating in series mode. res registers are controlled by a central control circuit UC and more specifically by

  signals UC] corresponding to quanta distances.

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  7. The outputs of the registers F] to F4 are connected to the inputs of a summing circuit CS which performs the addition of the information received on its inputs and supplies a result to a threshold circuit A]. The latter also receives a threshold signal SI1 of a determined value. The threshold circuit AI (or comparator) retransmits, to an inhibition circuit INH, the signals received from the summation circuit CS, if their value is greater than the value

the threshold signal S1.

  Furthermore, the circuit of FIG. 1 comprises a threshold level generating circuit S2 connected to the outputs of the IF registers. at F4. This circuit detects the value of the signal supplied by F1 to F4 having the highest value. From this value, it produces a threshold level signal (S2) whose value is lower than a known value, 6 dR for example, to that detected. Four threshold circuits All at A14, such as comparators, are connected to the outputs of the registers F1 to F4. They each receive the threshold level signal (S2) and retransmit on the inputs of an AND gate, a logic level signal "1", when the value of the sional received from the register FI to F4 is greater than the value of the signal. threshold level (S2). In the opposite case, the threshold circuits Ail 3 A] 4 provide a logic level signal "0". When all inputs to the AND gate are at the logical level

  "]", this gate provides an inhibition signal to the inhibition circuit INIH.

  If the AND gate provides a logic level signal "0", the muting circuit allows the transmission of signals received from the threshold circuit A],

  on the MEM link, to use circuits.

  On the other hand, if the AND gate supplies a logic level signal "1", the inhibition circuit prohibits this retransmission of signals on the link MEM. Under these conditions, it is assumed that the power of a target is constant during the transit time T of a

Radar beam on the target.

  As a practical example, it is assumed that during the illumination time

  T (approximately 80 ms), the recurrence frequency is switched four times, ie every T seconds (or all the recurrences where N is the

number of recurrences per plot).

number of recurrences per pad).

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  The conventional circuits perform, after Doppler filtering, normalization detection, a post integration adapted to the passage time T.

  According to the invention, the post-integration is replaced by 4 postintégra-

  successive occurrences of recurrences, each summation register F] to F4 being assigned to a particular recurrence frequency. The results of these four elementary summations are, on the one hand, summed in the circuit CS in order to realize the equivalent of the overall postintéqration on the time T and the result is presented at the threshold S1 of detection. On the other hand, each output of the summation registers is compared with a threshold S2 allowing

to develop the ai.

  When all ai are at 1, the echo is inhibited.

  Because of the transient spikes due to the switching of the recurrence frequency FR, the threshold S2 must be slaved to the level of the echo: the strongest of the 4 levels is thus detected and the threshold is set at 6 dB.

  under this value, with a S02 heel when the echo is weak.

  In these conditions, the filter equivalent qabarit is the corresponding one.

at -6 db.

  For weak echoes (close to the detection threshold), the previously calculated threshold S2 would be under the thermal noise. So we place a low stop at the evolution of S2 'The value of this stop must be

  as it minimizes the probability of errors.

  Two types of errors can be made: l / Wrong elimination of a useful target due to a false alarm resulting in the transition to I of a ai which would be zero without thermal noise. This probability of error is kept lower than

a few cents.

  2a / Non elimination of a slow target due to non-detection

  the maintenance at zero of an ai which should have been 1.

  The threshold S2 being fixed by the above criterion, the probability of not eliminating a slow target was calculated as a function of the power of this echo reported at the minimum detection power. -7 It can be seen that for echoes whose signal-to-noise ratio is

  greater than 4 dP, the elimination is better than 90%.

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  These probabilities are calculated from the instantaneous power in a passaqe. Overall probabilities should take into account

  the power fluctuation from one sweep to the next.

  To improve the performance of the system of the invention, a phase-change discrimination circuit is combined with the amplitude-variation discrimination circuit as shown in FIG. 2. This circuit is designed analogously to the discrimination circuit.

by amplitude variations.

  Call Ai = ni 'nor I the phase difference of the sional

  received between the recurrences n and n - 1, the recurrence frequency being F.

  q We have A i = F. d o fd is the noppler frequency of the

unambiguous target. -

  For FR = Fj, we have the same: Pl 27 f Ad, _ 2îf d J F. 4j = A <i - 6A j = 2fd - fT t> 1 i d F. FT5 i J

  the difference between the phase differences measured at frequencies Fi and Fj.

  It can be seen that the maximum of the 6AP ij 'makes it possible to discriminate, without any ambience, the echoes whose Doppler is less than 3000 Hz (slow echoes) of echoes whose Doppler is greater than 3000 Hz by placing a threshold r. However, this discrimination will be possible only if we can calculate the 6ó ij 'which supposes that there is only one echo per cell of resolution of the radar: it is thus necessary to be placed behind the Doppler filter to eliminate first the ground echoes. Under these conditions, dAA ij can not be

calculated only if we have aiet aj = 1.

  reci limits the scope of the device, to areas in which the calculation

is possible.

  However, the phase variation discrimination circuit joined to the amplitude variation discriminating circuit makes it possible to remove the doubt on the zones where all the ai's are at 1. and thus to remove certain holes from

  speeds created when eliminating slow targets.

  Fn placing a threshold at 670, we have a card of 20 for the calculation of the maximum of 6LA 4 'By calculation it is shown that the precision required

  is obtained when the sinal noise ratio () is greater than d.

  is obtained when the sional noise ratio (-) is greater than 3 dP.

  As has been said previously, the phase discrimination function is an improvement to the device of FIG.

the corresponding circuits.

  Such a device supposes moreover that one can calculate the phase of the signal, after filter rloppler, in each quantum of distance. It is therefore necessary to have two Doppler filters (one on each component of the

sianal) and a phase extractor.

  With these conditions, the phase-change discrimination circuit, as shown in FIG. 2, comprises: two Doppler filters FF. and FDy for the two components ex and Ey of signal; two circuits for calculating phases f ni and A +. connected to the two FDx and FDly Doppler filters; a switch C.3 whose input is connected to the circuit A. and making it possible to connect this input to one of its parts 31 to 34; the summation registers (1 to G4) of the phases A + connected to the outputs 31 to 34 of the switch C3;

  a search circuit for the maximum phase variation 6A.

  connected to the outputs of the summation registers and supplying the value of this maximum on an output; a threshold circuit connected to the output of the preceding circuit 6AQ, comparing the value of the signal present on this output with a threshold S3 and supplying an inhibition signal when the value of the signal is below the threshold S 3. The value of threshold can be according to the previously taken example of 67; a gate ET2 whose first input is connected to the output of the preceding threshold circuit and the second input is connected to the output of a gate ET1 corresponding to the AND gate of the circuit of the

figure 1.

  FIG. 2 shows the amplitude variation discrimination circuit of FIG. 1. The results of discrimination by amplitude and phase variations are combined by the gate FT2 which provides an inhibition signal to the output circuit. INH if a slow mobile is detected in the

  conditions previously described.

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  1]. Referring to FIG. 3, a variant of an exemplary embodiment of the discrimination discrimination circuit will now be described.

of amplitudes of the invention.

  These circuits are divided into two groups: - a group that operates at the rate of the quantum distance and for each recursion under the control of signals UCi provided by the unit of

UC command.

  - a group that only works recurrence corresponding to a change of frequency, under the control of UC2 signals provided by

the IJUr control unit.

  The first circuit group mainly comprises 4 registers h offset F1 to F4 of n boxes and p bits where one is the number of distance cells of the domain (limited for example to 254) and p the number of bits necessary to perform the integration of the envelope detector output on 20 recurrences representing a quarter of the time the beam passes on the target. Two multiplexers or switches C1 and C2 connect the input and the output of a register (Fl to F4) on a summator CS3 during the recurrences for which FR = Fi. For the first recurrence of the cycle F. = Fi, the input is set to zero by I3, which empties the contents of the register (i). Then the incidental information is summed in this register, until FR = Fi + 1 (modulo 4). At this moment, the information of the 4 registers, corresponding to the last four sequences Fi 3 Fi - 2 Fi] Fi, are transferred in the second group of circuits (at the same time as the

  register (i + 1) = (i - 3) is reset to zero).

  This second group of circuits consists essentially of

  circuits connected at the output of registers F1 to F4.

  The information that will be called g i - 3 'i - 2' i - 1 Z i provided by the registers F1 to F4 are combined in a circuit represented by a

summing circuit CSl.

  For the understanding of the present device, it is considered that

  a sum, adapting the postintéqration to the time of passage on the target.

  Then, the information thus summed is compared with a threshold S. (circuit A1) fixed by the desired conditions of false alarm and the result

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  is memorized, after passace in a door, in a decalaate register R1

of n boxes of 1 bit.

  Three other identical registers R2 to R4, placed in series,

  The last 3 information streams corresponding to the sequences are stored in memory: i-1, i-2, i-3, i-4 (-1, i-2, i-3, 4-F] - 2 'Fi 3' Fi) i-2, i-3, i-4, i - 5 (FF 3 Fi F _ 1) i - 3 -4, -, - 6 (i-3, Fi _ Fi 2 )

  The information E i 'i -' i -2 'Z i -3 are, on the other hand,

  to the circuit developing threshold S2.

  This circuit takes the maximum of Z j (ie Z p), divide it by 2 (6 dR is a shift of 1 bit) and the value E p is compared to threshold S2. If E p Ep 2 9s

2 SO2 then 52 = otherwise 52 = S2.

  ZE j are compared to 5S in circuits A] 1 to A 14, to provide the a. whose lonic product, inverted by a NAND gate, controls the circulation of information in the preceding registers. When all has. are at 1, all the information corresponding to this target (which is present J

  in the sequences, i, i - 1, i - 2, i - 3) is erased.

  Otherwise, the information is found at the exit of the 4th reoistre with a

  delay corresponding to a beam width.

  It should be noted that if there is a slow target, sequences i - 4, i - 5, i - 6 corresponding to 3 additional beam are also erased, and we will see a "shadow" side of the vehicle echoes.

Claims (5)

  1. Doppler radar slow mobile echo elimination circuit comprising antenna reception circuits (El) providing on an output link (.0) the digital value of the detected signal, characterized in that it comprises a first switch (C].) having an input connected to the link (10) of the reception circuits E1 and a number of outputs (1 to 14) equal to the number of different recurrence frequencies of the loppler radar, said switch (C1) for connecting its input to the output (1 to 14) corresponding to the current recurrence frequency; a summation register (F1 to F4) connected to each output (11 to 4) and supplying on an output a detected sienal value: a summing circuit connected to the different outputs of the summation registers (FI to F4) performing a post-integration on the antenna beam passing time on the target of the detected signals to provide on an output an inturning signal - a coincidence circuit (ET) having as many inputs as there a value of the recurrence frequency, each of these inputs being connected to an output of a summation register (F1 to F4), so that the coincidence circuit (ET) provides an inhibition signal on an output of inhibition when each summation request delivers a signal of level higher than a determined threshold (S2) - an output circuit (INH) having a first input connected to the output of the summing circuit, a second input connected to the output of d ' inhibition of the coincidence circuit (AND), this output circuit (INH) allowing, in the absence of an inhibition signal on the second input, retransmitting any integration signal received on the first input, on an output to display circuits, this circuit (INH1) also making it possible to prohibit the retransmission of the integration signal in the event of presence of a muting signal on the second input:   - and a control circuit (U (.) providing signals of com-   to the switch, summing registers and summing circuit. 2.5 7 1 150   ] 4 2. Slow moving mobile echo elimination circuit for noppler radar, according to   claim 1, characterized in that it comprises a second switching   (C2) having as many inputs (21 to 24) as there are values of the recurrence frequency, each connected to an output of a summation register (F1 to F /), and an output connected to a first input of an addition circuit (CS3), which has a second input connected to the output of the reception circuits (El) and an output connected to the input of the first switch (C1).   Doppler radar slow moving echo elimination circuit according to claim 1, characterized in that each summation register output (F1 to F4) is connected to a coincidence circuit by   via a threshold circuit (A1] to A14).   4. Doppler radar slow mobile echo elimination circuit according to claim 3, characterized in that it comprises a threshold level generating circuit (S2) connected by its inputs to each output of the summation registers ( F1 to F4) and supplying, on an output connected to the threshold circuits (All to A14), a threshold signal whose level is determined from   of the signal having the maximum level on its inputs.   5. Doppler radar slow mobile echo elimination circuit according to claim 1, characterized in that the reception circuits (El) have two Doppler filters (FDX, FD y) giving the values of two components of the received signal. , phase difference measuring devices (ni, A4 i) connected to the two Doppler filters (FDX, FD y) and supplying at the input of a third switch (C3) a digital signal of phase shift value, said switch (C3 ) having as many outputs (31 to 34) as there are values of the recurrence frequency, each of these outputs being connected to a phase summation register (Gi to G4), the outputs of the summation registers (G1 to G4) are furthermore connected to a phase variation maximum search circuit (6 A ij) whose output is connected to a threshold circuit (A3) providing on an output an inhibition signal when the value of the received signal (phase shift) is less than one determined value, said output of said threshold circuit being connected to an input of a two-input AND (ET2) type digital gate and an output whose other input is connected to the coincidence circuit inhibiting output ( AND) providing an inhibition signal to the INH output circuit   when he receives a sicgnal on each of these entries.